In 1942, Italian physicist Enrico Fermi led a team of forty-eight scientists at the University of Chicago who built the first artificial nuclear reactor, known as “Chicago Pile-1,” for the Manhattan Project. After dozens of modifications to the pile’s design, on December 2, 1942, Fermi’s group achieved the first controlled, self-sustaining nuclear chain reaction. Their research was essential to the construction of the atomic weapons used in World War II and the development of nuclear power after the war.
Chicago Pile-1 would be unrecognizable as a nuclear reactor today. Built in an abandoned squash court under the west stands of old Stagg Field at the University of Chicago, the pile was composed of 40,000 graphite blocks that enclosed 19,000 pieces of uranium metal and uranium oxide fuel. Originally conceived of as a sphere, the reactor took shape as a flattened ellipsoid. The graphite was organized in layers in a 24-foot-square frame of timbers. Its purpose was to slow down the free neutrons produced by the uranium, increasing the chances that the neutrons would be absorbed by other uranium atoms and generate nuclear fission. Fermi called the reactor “a crude pile of black bricks and wooden timbers.”
Fermi's secret project developed over many months. His team had to rearrange the pile thirty times before the final test of the system was ready. Working in twelve-hour shifts in an unheated room, the team cut and moved the graphite blocks in intricate patterns to determine the optimum configuration for the reactor. Looking like coal miners, they left work each day covered in black graphite dust. Fermi directed and monitored the number of uranium neutrons in circulation as the layers were added. He used cadmium rods to control the movement of uranium neutrons in the pile, awaiting the right time to release this control and allow the neutrons to generate an exponential chain reaction on their own.
Initially, this project was supposed to be conducted in the Red Gate Woods south of Chicago, but a labor strike on that isolated site led Fermi to move the project to Stagg Field. Fermi’s calculations convinced him that the risk of a genuine nuclear explosion were very low; nevertheless, his team built an unshielded nuclear reactor in the middle of one of America’s most densely populated cities. The worst case scenario might have been a meltdown of the pile, with the uranium catching fire and spraying radiation.
The pile went critical on December 2, 1942, when Fermi directed a young scientist named George Weil to remove the final cadmium control rod as a group of dignitaries watched. With this check on the uranium neutrons removed, the pile was able to achieve a sustained chain reaction for twenty-eight minutes. The neutron counters monitoring the pile slowly began to make their distinctive clicks and clacks, picking up speed until they generated a roar that signaled the experiment’s success.
In February 1943, the reactor was dismantled and moved to Red Gate Woods, where it was reconstructed, shielded, and renamed Chicago Pile-2. Eventually, it was closed and buried on site. The site of Chicago Pile-1 was designated a National Historic Landmark in 1965 and added to the National Register of Historic Places a year later.